Hybrid microcircuits are characterized by microcircuit elements, e.g., conductors, resistors and capacitors, that are deposited as thick films on a substrate, such as ceramic. Thick films are typically about 0.5 mil (about 12.5 .mu.m) or more in thickness, and often formed by screen printing a paste or ink composition that is then dried and fired. Thick-film pastes generally contain an organic vehicle in which is dispersed in an inorganic particulate filler, which is electrically conductive, resistive or dielectric, depending on the circuit element desired. The particulate filler is generally in the form of powdered solids, though in some cases the particulate filler may be dissolved, and becomes a solid during firing. Firing the thick-film paste serves to burn off the organic vehicle and sinter the particulate filler.
To obtain flow properties appropriate for screen printing, thick-film pastes typically contain about 35 to 45 volume percent powdered solids, with the balance being organic vehicle. Depending on the density of the powdered solids, this volume fraction may correspond to about 70 to 90 weight percent solids. Factors that affect the flow properties of a thick-film paste include solids content, the viscosity of the organic vehicle, and the shape and average size of the solid particles. Solids content is particularly critical for achieving a beneficial pseudoplastic property to the rheology of the paste, by which the paste exhibits a relatively high viscosity at low shear rates, and lower viscosities as the shear rate increases. If the solids fraction of the paste is increased significantly above an acceptable range, the paste becomes too viscous and elastic to print well. On the other hand, if the solids fraction is decreased significantly below this range, the paste is not sufficiently tacky and cohesive to print well.
Because most thick-film pastes have a similar solids loading, the dried film thickness of different pastes printed through a given screen will typically fall within a similar range, e.g., about 0.7 to 0.9 mils (about 18 to 23 micrometers) when printed through a 290 mesh screen. The fired film thickness of a paste is generally equal to the dried film thickness times a densification factor, which typically ranges from about 0.5 to 0.8. For some applications, minimizing the fired film thickness of a printed paste is beneficial. However, achieving a significant reduction in fired film thickness without negatively affecting print performance (e.g., line definition) of the paste can be difficult. If the solids fraction of a paste is decreased in order to permit printing of thinner films, the paste will be excessively fluid unless the organic vehicle is either modified or changed to compensate for the lower solids content. However, simply increasing the viscosity of the vehicle to compensate for the reduction of powder can also cause problems. If the organic vehicle lacks a significant elastic component to its rheology, the paste solids will tend to settle out and agglomerate. Increasing the viscosity sufficiently to prevent settling will result in a paste that is too viscous to print. Thixotropic additives that impart a significant elastic component to organic vehicles result in unstable rheology; after resting, the paste will be extremely stiff, but while the paste is being worked the viscosity and elasticity continuously drop as the organic structure breaks down.
An example of an application that is complicated by the circumstances described above is illustrated in FIG. 1, which shows a circuit component 10 electrically connected to a conductor 12 with a solder joint 14. Also shown is a solder stop 16 formed with a conventional dielectric paste. The function of the solder stop 16 is to render part of the surface of the conductor 12 unwettable by the molten solder that forms the solder joint 14 during reflow, and thus defines the solderable areas of the conductor pattern. The component 10 is also shown as being underfilled with an adhesive 18 to promote reliability. In such applications, it is important to minimize obstructions to the flow of the adhesive 18 during the underfill process. Obstruction of the underfill adhesive 18 can result in an incomplete fill under the component 10, which is difficult to detect and may result in an unreliable part. Therefore, it is desirable that the fired film thickness for the solder stop 16 is as thin as possible while still being sufficient to prevent wetting of the conductor 12 by the molten solder. Theoretically, a solder stop could be less than one micrometer thick and still prevent wetting of the conductor 12.
Another consideration of solder stop performance is that it should be printed with good definition, since failure to resolve the desired solderable surface on the conductor 12 would result in a non-functioning or unreliable component 10. Pastes which have sufficient solids loading to achieve the necessary definition typically contain 70 to 80 percent solids by weight, but result in a fired film thickness of about 7.5 to 15 micrometers using standard thick-film processes and tools. This is thicker than necessary for rendering the conductor surface unwettable, and therefore unnecessarily impedes the underfill adhesive 18 without offering functional benefits. Attempts to reduce fired film thickness with pastes having lower solids content have resulted in inadequate line definition.
Similar problems are encountered if attempting to reduce the fired film thickness of thick film conductors, resistors and capacitors. Accordingly, essentially all thick-film processes could benefit from a thick-film paste that can be printed to achieve a reduced thickness while maintaining printing performance, including line definition.